More particularly still, it relates to means for extruding an elongated product of any section, but especially one of hollow section, having a bent or sinuous configuration. One known method of extruding a product of hollow section to such a configuration is described for instance in an article on pages 38-39 of "Elastomerics", March 1986, and essentially requires apparatus, referred to in the article as a "moving head extruder", in which the inner member of the die can move out of concentricity with the outer member in a carefully controlled manner. When the inner and outer members of the die are not concentric, so that they define between them an uneven annular gap that is thicker to one side of the extrusion axis than the other, extruded material leaves the die at an uneven speed, the parts of the periphery which correspond to the thinnest and thickest parts of the annular section travelling respectively slowest and quickest. This tends to induce in the product a bend in the axial plane which includes the two but displaced axes of the inner and outer die members, the outside of the bend corresponding to the part of the periphery where the material was travelling fastest on leaving the die. Obvious limitations of such a known process are that it applies only to dies which extrude a hollow product and which therefore include an inner die member, and also that the die is of variable geometry and the mechanism for supporting the inner member and controlling its translational movements relative to the outer one are necessarily relatively complex and costly. Also, such a process can only apply where the extrudate on leaving the die is not yet solid, but still strictly a very viscous fluid.
An example of another known kind of method for bending an extruded product is to be found in Patent Specification U.S. Pat. No. 3,748,077. In this example, when the leading end of the extrudate emerges from the die, it is received by a collar or clamp mounted at the free end of a pivoted arm. As the extrudate then continues to emerge from the die, the region engaged by the cap or clamp is constrained by the arm to follow a circular arc, so forming the extrudate into a bend. In such a process the entire force used to create the bend has to be provided by the extrusion ram, and the radius and direction of the bend are both fixed by the length of the arm and the location of its pivot. Also, although the cap or clamp is constrained to follow a circular arc, there is no guarantee that the final shape of the extrudate, following behind it, will conform accurately to the same arc.
Our invention arises firstly from appreciating that a variation, around the periphery of the product, of the velocity of the extrudate leaving a die may in appropriate circumstances be achieved more simply than as described in the "Elastomerics" article, in a way applicable to both hollow and non-hollow extrudates, and with a fixed-geometry die. Secondly, from appreciating that where a fixed-geometry die is used and bends are created by applying a constraint to the product after its surface has become solid, adequate control of both the radius and the direction of the bends requires that a second force, in addition to the extrusion force, must be generated and must act upon the extrudate so as to perform work upon it, over and above the work performed upon it by the extrusion force and upstream of where it becomes solid. Our invention is defined by the claims, the contents of which are to be read as included within the disclosure of this specification, and includes methods and apparatus as described with reference to the accompanying drawings.